Issue with serialization, __sleep method and field defined in trait

Hi, our team faced strange issue with HHVM 3.4.2:

```
fieldOne = 1;
$var->fieldTwo = 2;

var_dump(unserialize(serialize($var))->fieldTwo, $var->fieldTwo);

// expected output
//int(2)
//int(2)

// actual output
//NULL
//int(2)
```